The present invention relates to a tilt detecting device equipped with an electric bubble tube, and in particular, to a tilt detecting device for precision instrument such as survey instrument equipped with a light transmission type electric bubble tube.
The instrument such as survey instrument is generally installed at a predetermined place during surveying, and the state of installation of the instrument is adjusted to a reference position in each surveying operation according to a tilt detecting device. The survey instrument forms a reference line, a reference horizontal plane, etc., and the adjustment of the reference position must be performed with high accuracy.
In the following, description will be given on a survey instrument equipped with a tilt detecting device.
As one type of survey instruments, there is a laser survey instrument. The laser survey instrument forms an irradiation plane by projecting a laser beam with directivity for rotary irradiation in the horizontal direction. A photodetector (not shown) arranged on the rotary irradiation plane receives and detects the laser beam, and a reference line and a reference plane are obtained.
Brief description will be given now on the laser survey instrument as described above referring to FIG. 13.
In FIG. 13, reference numeral 1 represents a laser beam emitter, and it is supported in such a manner that it can be tilted in any direction, and a rotator 2 rotatable around an optical axis of the laser beam emitter 1 is provided on its head. The laser beam emitter 1 comprises tilt sensors 3 and 4 (transmission type electric bubble tubes) in two horizontal directions, which are perpendicular to each other, and a tilt sensor 5 in the vertical direction. The tilt sensor 5 and the tilt sensors 3 and 4 as well as a tilt detection controller (not shown) constitute a tilt detecting device.
The rotator 2 deflects the laser beam emitted in the vertical direction to the horizontal direction and is rotated by a scanning motor 6, thereby performing rotary irradiation of the laser beam 7.
From the laser beam emitter 1, arms 8 and 9 (arm 9 is not shown in the figure) are extended in two horizontal directions, which are perpendicular to each other, and tips of the arms 8 and 9 are engaged with a tilting mechanism.
The tilting mechanism comprises two sets of tilt driving units 16 and 17 (tilt driving unit 17 is not shown) provided with respect to the arms 8 and 9 respectively and a tilt control unit (not shown) for controlling the tilt driving units 16 and 17. Each of the tilt driving units 16 and 17 comprises a screw 11 extending in the direction of the optical axis of the laser beam emitter 1, a nut 12 screwed on the screw 11 and in contact with tip of the arms 8 or 9, and a tilt adjusting motor 15 for rotating the screw 11 via gears 13 and 14. In the figure, reference numeral 18 represents a focus adjusting device, which performs focusing of the laser beam 7 by moving a condenser lens 19 arranged in an optical path of the laser beam emitter 1 in the direction of the optical axis.
When the laser beam 7 is projected in the horizontal direction from the rotator 2 and the rotator 2 is rotated by the scanning motor 6, an irradiation plane is formed, and when the laser beam 7 scanning position is set at a predetermined position by the photodetecting device, a reference plane is obtained.
When the reference plane is obtained, it is possible to easily determine working position in extensive range, e.g. for setting of window position in interior finishing work during building construction or for ground leveling in civil engineering work.
Each of the tilt sensors 3, 4 and 5 of the tilt detecting device is based on a combination of a light emitting element, a photoelectric conversion element and a bubble tube. In the following, description will be given on conventional type tilt sensors 3, 4 and 5 referring to FIGS. 14 to 16, and an general feature of the tilt detecting device will be described referring to FIG. 17. The tilt sensor 3 has the same structure as the tilt sensors 4 or 5, and description will be given here only on the tilt sensor 3.
A light emitting element 22 such as LED is arranged in a direction perpendicular to an axis of a bubble tube 21, and a pair of photodetector elements 23 and 24 are arranged at opposite positions to the light emitting element 22 with the bubble tube 21 therebetween and at symmetrical positions with respect to an optical axis of the light emitting element 22 with a predetermined distance between them. Signals from the photodetector elements 23 and 24 are inputted to a tilt detection controller 25.
Of detection light 26 emitted from the light emitting element 22, central light components are diverged when passing through an air bubble 27 of the bubble tube 21, and peripheral components of the detection light 26 except the central components are converged by the bubble tube 21 and reach the photodetector elements 23 and 24. The photodetector elements 23 and 24 detect the tilt from amount of received light components of the detection light 26. The amount of the light passing through a portion with the air bubble 27 is different from the amount of the light passing through only a liquid portion 28.
As shown in FIG. 17, the tilt detection controller 25 comprises a comparison arithmetic unit 30 where photodetection signals from the photodetector elements 23 and 24 are inputted, and a controller 31 for issuing a control signal based on a signal from the comparison arithmetic unit 30. A driving circuit 32 drives the tilt adjusting motor 15 based on a control signal from the detection light 26.
First, description will be given how the detection light 26 passes through the bubble tube 21 referring to FIGS. 14 to 16 and FIG. 18.
FIG. 14 shows how the detection light 26 passes through the bubble tube 21 in the longitudinal direction. Because the detection light 26 passes through the air bubble 27 and the liquid portion 28 almost in the straight direction, the detection light 26 is received by the photodetector elements 23 and 24. At boundary portion between the air bubble 27 and the liquid portion 28, the detection light 26 is reflected and it is not received by the photodetector elements 23 and 24. FIG. 18 shows amount of light components of the detection light 26 passing through in the longitudinal direction of the bubble tube 21. The portion with the least amount of light represents the boundary portion between the air bubble 27 and the liquid portion 28.
FIG. 15 is a cross-sectional view of the air bubble 27 of the bubble tube 21. The light straightly passes through the bubble tube 21 and reaches the photodetector elements 23 and 24. Except the central portion of the air bubble, the components detection light 26 passing through the air bubble 27 is not converged but it is diverged although it depends on radius of curvature of the air bubble 27 and the bubble tube 21. The components of the detection light 26 passing through the liquid portion 28 around the body of the bubble tube 21 are converged toward the photodetector elements 23 and 24 by optical effect. The components of the detection light passing through boundary surface of the air bubble are mostly reflected due to refraction and are not received by the photodetector elements.
FIG. 16 is a cross-sectional view of the liquid portion 28. The components of the detection light 26 passing through the center of the bubble tube 21 pass through straightly and reach the photodetector elements 23 and 24. Passing through the liquid portion 28 of the bubble tube 21 except the central portion, the detection light 26 components are similarly converged to the photodetector elements 23 and 24 by optical effect. The photodetector elements 23 and 24 are designed in such a manner that they have substantially small widths to prevent receiving of noise light. Because the photodetector elements 23 and 24 have small widths for photodetection, only the detection light 26 components passing through the bubble tube 21 are received, and this increases photodetection contrast.
FIG. 18 represents change of amount of light along the axis of the detection light 26, which passes through the bubble tube 21. At the boundary of the air bubble 27, the detection light 26 is reflected, and amount of transmitting light is extremely low. By passing through nearby the boundary of the air bubble 27, the optical path of the detection light 26 is changed and its reflection increases thereby the amount of light components of the detection light 26 passing through the bubble tube 21 is reduced. Accordingly, the range G between two boundaries of the air bubble 27 is the range where the amount of light is reduced. When the air bubble 27 is moved, the range G of the low transmitting light amount is also moved. Thus, photodetection amounts of the photodetector elements 23 and 24 are changed, and tilt can be detected according to the change in the output of the photodetector elements 23 and 24.
As shown in FIG. 14, in case the bubble tube 21 is at the horizontal position, the air bubble 27 is at the center of the bubble tube 21, and amounts of the detection light 26 entering the photodetector elements 23 and 24 are equal to each other. Therefore, after comparison operation at the comparison arithmetic unit 30, there is no deviation between signal from the photodetector element 23 and signal from the photodetector element 24, and no driving signal is issued to the driving circuit 32.
Next, if the laser survey instrument is tilted and the air bubble 27 of the bubble tube 21 is moved rightward in FIG. 14, the components of the detection light 26 entering the photodetector element 24 are discarded, and components of the detection light 26 entering the photodetector element 23 are increased. Accordingly, there occurs a difference in the photodetection signals inputted to the comparison arithmetic unit 30. The resultant deviation signal is inputted to the controller 31. From the controller 31, a control signal is issued to the driving circuit 32, which drives the tilt adjusting motor 15 according to the control signal until the difference of photodetection signals from the photodetector elements 23 and 24 is eliminated.
As the photodetector elements 23 and 24, photosensor, CCD, or linear sensor may be used.
As described above, within the range G of the low transmitting light amount, the detection light 26 passing through the air bubble 27 or the detection light 26 converged near the body of the bubble tube 21 enters the photodetector elements 23 and 24. Accordingly, as seen in FIG. 18, there is a certain photodetection amount in the range G. In the conventional type tilt detecting device as described above, tilt is detected according to the difference between the light amount in the range G and the light amount on both sides of the range G. In this respect, if there is any photodetection amount in the range G, detection accuracy is decreased. In case the difference of photodetection is small, it is often difficult to detect the tilt. If the photodetector element is designed with narrower width, it is disadvantageous because the photodetection amount is reduced.